Alpine 3.15 possible regression: Postfix built without EAI/SMTPUTF8 support

Postfix has long supported EAI/SMTPUTF8 (http://www.postfix.org/SMTPUTF8_README.html). Postfix's build system will automatically decide whether to enable this functionality by attempting to detect the ICU library. See https://github.com/vdukhovni/postfix/blob/c4754532ffd9bdcb9a0ccb67973cbe93412807fb/postfix/makedefs#L801-L842.

In Alpine 3.13 and 3.14, Postfix was built with EAI/SMTPUTF8 support. See, eg, https://build.alpinelinux.org/buildlogs/build-3-13-x86_64/main/postfix/postfix-3.5.12-r0.log and https://build.alpinelinux.org/buildlogs/build-3-14-x86_64/main/postfix/postfix-3.6.3-r0.log.

In Alpine 3.15, however, Postfix is built without EAI/SMTPUTF8 support. This causes Postfix to lack support for non-ASCII headers. See https://build.alpinelinux.org/buildlogs/build-3-15-x86_64/main/postfix/postfix-3.6.3-r0.log and note presence of -DNO_EAI -DDEF_SMTPUTF8_ENABLE=\"no\".

I don't see any relevant recent changes in the APKBUILD, but perhaps something was previously pulling in ICU and is no longer. I suggest adding icu as a build-time dependency for Postfix.